Wave Energy Watercraft

ABSTRACT

A watercraft comprising a plurality of linearly ordered, connected segments that are configured to enable the connected segments to articulate relative to each other in at least one degree of freedom. The watercraft harvests mechanical energy from ambient waves by means of the articulation of the segments, and converts the mechanical energy into electrical energy that powers a propulsion mechanism.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional patentapplication No. 61/800,374.

BACKGROUND

There is an increasing need for unmanned, autonomous sensor platformsfor use in the maritime environment that support monitoring operationsfor very long durations without human intervention. These platforms needto communicate, navigate and maneuver to position sensors at desirablelocations, water depths, and times. Long duration missions require thevehicle to carry or obtain the energy necessary for locomotion, sensorpower, and communications.

SUMMARY

An apparatus, comprising a plurality of linearly ordered segments, theplurality having two extremity segments, and wherein at least twosegments are connected; a connecting assembly joining each of the twoconnected segments of the plurality of linearly ordered segments, theconnecting assembly configured to enable the two connected segments toarticulate relative to each other in at least one degree of freedom; awave energy harvesting mechanism operatively connected to the connectingassembly that extracts mechanical energy from ambient liquid waves andconverts the mechanical energy into electrical energy; a propulsionmechanism attached to an extremity segment, operatively connected to andpowered by the wave energy harvesting mechanism.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts an embodiment of the invention in 7 linear segments.

FIG. 2 depicts an embodiment of the invention wherein wave energyharvesting is by means of hydraulic pumps.

FIG. 3 depicts a wave energy harvesting mechanism employing a geartrain.

FIG. 4 depicts an embodiment of the invention wherein wave energyharvesting is by means of linear Lorentz style generator.

FIG. 5 depicts a wave energy harvesting mechanism employing a circularLorentz style generator.

FIG. 6 depicts a close up view of the coil rings of a circular Lorentzstyle generator.

FIG. 7 depicts a wave energy harvesting mechanism employingpiezoelectric elements and flex-tension amplifiers.

FIG. 8 depicts the propulsion mechanism of an embodiment of theinvention.

FIG. 9 depicts the attenuation of water velocity with depth for a wave.

DETAILED DESCRIPTION

The embodiment described in the above summary solves the problem ofproviding a means for long duration, watercraft, sensor platforms byextracting energy from ambient water waves, converting the wave energyinto electricity, and then using that energy for locomotion, sensorpower, and communications. Excess energy is stored onboard the vehiclefor use when ambient wave energy is insufficient for operations. Theembodiments described in detail below are comprised of multiple segmentsflexibly connected together. Electric energy is produced by differentialmotion of adjacent segments resulting from wave-induced motion of thewater. Relative motion between adjacent segments can be converted toelectricity by a number of means; electromagnetic means (magnets andinductors), pump fluids to spin electric generators, or strainpiezoelectric elements to produce electricity. Power generation may beaugmented with photovoltaic elements attached to the upward-facingsurfaces of the vessel.

As shown in FIG. 1, embodiments of the invention may be composed of oneor more segments 110 connected with one or more degrees of freedom. Thesegments are able to move relative to one another in concert with thewave motion of their environment. In this way, certain portions of twosegments will alternatively move closer together and further apart inaccordance with the save motion. The energy of this relative motioncaused by waves can be captured by wave energy harvesting mechanisms120. The segments can sit at varying levels with respect to waterline130, including completely submerged. Individual segments may have lengthto width ratios (“L/W”) near unity or they may be elongated. Thecross-sectional shape could be of any geometric design such as circular,square, triangular, oval or any other symmetrical or asymmetrical shape.For instance a segment could be spherical in low length to width ratiosor cylindrical in shape for large L/W ratios. It could be cubic in lowL/W ratio or rectangular prism in large L/W ratios. Embodiments of theinvention may be composed of a single line array of connected segmentsor multiple arrays of segments to form two or three dimensional shapesof any desired configuration.

Techniques for conversion of wave energy to electric power are known inthe art. For example, the Pelamis system manufactured by Pelamis WavePower Limited provides off shore power generation by means of a linearray of multiple floating segments is anchored is a wave field. See forexample “Pelamis Technology” and “Pelamis Wave Power Brochure” bothpublished by Pelamis Wave Power Limited and both of which areincorporated by reference into this specification.

Wave action will typically align the array to be at a large angle to thewave crests. As waves move under the Pelamis system, segments are tilteddifferentially to conform to the wave shape. The segments are movablyconnected so that segments adjacent to the connection can be atdifferent angles when projected into a vertical plane. The connectionpoint may also allow the segments to move differentially in thehorizontal plane. Consequently, the space between adjacent segmentsopens at some points and closes at others as a wave passes across thesegments. When the wave crest is positioned at the connection pointbetween segments, the gap between segments is opened at the top of theconnection point and closed at the bottom. As the trough of the wavemoves past the connection point, the gap at the top of the connectionpoints is closed and the bottom is opened.

As shown in FIG. 2 and in the Pelamis system, a wave energy harvestingmechanism can be implemented by means of hydraulic pumps. The cyclicopening and closing of space between segments actuates hydraulic pumps210, which are integrated with a connecting assembly 240, joiningsegments together. One end of the connecting assembly is attached to afirst segment and another end of the connecting assembly is integratedwith the hydraulic pump piston and attached to a second segment. Withappropriate valves, piping and hydraulic accumulators 220, this waveenergy harvesting mechanism spins a rotary hydraulic motor which turnsan electrical generator 230. See for example, U.S. patent applicationpublication number US20130239562 A1, which is incorporated by referenceinto this specification. The electrical generator is operativelyconnected to propulsion mechanism 270 attached to an extremity segmentof the watercraft.

A wave energy harvesting mechanism system employing hydraulic pumps iscapable of producing 750 kW of power with 70% efficiency of itspower-take-off system. The wave energy flux of water in waves isestimated by the equation

${P = {\frac{\rho \; g^{2}}{64\; \pi}H^{2}T}},$

where H is the wave height (trough to crest), T is the wave period, ρ isthe density of water and g is the acceleration due to gravity. For amodest sea offshore characterized by 2 meter high waves with awavelength of 80 meters, a period of 7 seconds and velocity of 11.1 m/s,the potential power is about 13.4 kW/m of wave crest.

One means of transforming the mechanical forces of wave motion acting onmultiple hull sections is to convert the flexing moment of the hullsections into rotary motion by means of a gear train as illustrated inFIG. 3. Adjacent hull segment are connected at a flexible joint 310allowing at least one degree of freedom. In this embodiment, the hullsare allowed to flex in the vertical plane. Other arrangements of theflexible connection could allow for flexure in the horizontal plane andother dimensions as well. For simplicity and clarity, a single degree offreedom in the vertical plane will be used to describe theimplementation.

A segment of a circular ring gear 320 affixed to Hull Segment D isconcentric with the pivot point of the hull segments C and D. The ringgear is engaged with a smaller gear 330 mounted on Hull Segment C whichis coaxially connected to a larger diameter gear which drives one ormore additional gears to increase the angular velocity. The overall gearratio of each gear and combination of gears in the gear-train 340 can beselected to accommodate an average flexure (for and average sea height)between hull segments and the angular velocity best suited for aparticular rotary electric generator 350. Amplification of angularvelocity will typically be on the order of hundreds to thousands.

Because the direction of flexing changes periodically with the passageof each wave crest and trough, the flexural amplitude changes with seastate. The design of the gear-box and vehicle characteristics can betuned for maximum efficiency for a particular sea state or changedynamically to accommodate a range of changes in sea state. Efficiencymay be optimized by converting bidirectional flexural input tounidirectional output. Conversion of the flexural motion could also beconverted to output constant velocity rotational output. Electricalpower generated will be conditioned to achieve the power type andvoltages needed by propulsion hardware, navigation, communication andpower storage equipment.

Provided that the generator can also be used as a motor, it and the geartrain can be used in combination with position sensors mounted on thehull segment to position the hull segments in desirable configurations.Examples of desirable configurations might be those advantageous formaneuvering or to synchronize movement of the multiple jointssimultaneously to mimic the swimming motion of a water snake. It mayalso be desirable to drive a small diameter rotary electric generator ora larger diameter circular motor similar to those illustrated in analternative embodiments described below.

For illustration purposes, it is useful to provide an example of onemethod for estimating the power generated by an embodiment of theinvention. Returning to FIG. 1, three power modules may be connected oneither side by rigid hull sections 1 meter in diameter and about 7 mlong. Each power module is free to bend to conform to the wave surfaceas it passes by.

Each 7 m segment has a hull 5 m long and thus a displacement of about of3925 kg if neutrally buoyant. Assuming 20% positive buoyancy, thisembodiment produces a displacement of 3142 kg. Distance from center ofpower module to center of gravity for the adjacent rigid section is 3.5m.

Torque (T)=force*lever arm. Therefore, T=3142 kg*9.8 m/s2*3.5 m=107,770Nm. Power is torque multiplied by the angular velocity of action.

In the sea state illustrated, each hull segment sweeps through 2.5degrees up and down (or 5.0 total) every 7.1 seconds as the crest andtrough moves under each power module. Therefore, P=(T/7.1 s)*5.0deg*6.28 rad/360 deg=9,400 J/7.1 s.

Each power module has a rigid hull section either side of it so power isdoubled for each power module. Therefore, P=18,800 J/7 s=2,686 watt foreach power module.

All three power modules produce 8,058 W or 10.8 hp if all of the powerat the power module was converted to electricity. Assuming an efficiencyof power conversion of 30%, 3.25 hp would be available for systemoperation (e.g., propulsion, on-board system use and power storage).

Speed to length ratio (“S/L”) for a displacement hull is estimated usingthe empirical relationship S/L=(1200/((Disp/hp))̂(−3). Total displacementis estimated to be 5(3,142 kg) or 34,562 pounds. Therefore, S/L isestimated to be 0.48.

For a displacement hull with a waterline length of 90 ft (as describedin FIG. 5) operating at the surface, the expected speed of the vessel inthe described sea-state is 4.6 kt. Because of the large length to beamratio, the speed may be greater. When the vessel is submerged, theresistance of cutting the air-water interface and wave-drag are reducedand the speed may be greater yet.

The angle of attack with respect to the vessel and the wave crest alsoaffects the conversion factor of wave energy into electrical power. Thescenario described above assumes the vessel was operating perpendicularto the waves. The direction of travel affects the power conversion too.Propelling the vessel in the direction of the waves reduces the observedperiod of the wave and so reduces the power extracted. Heading thevessel parallel to the wave crest will reduce the power converted.

The operating efficiency of a multi-segmented wave energy extractionsystem can be optimized for the dominant sea state of the operatingarea. Segments too long in short wavelength seas will not be asefficient as shorter segments. Bending angle between adjacent segmentswill be maximized when segment lengths are one quarter of thewavelength. Use of many short segments increases the number ofconnection and power generation components that comprise the same lengthof array and also increases the labor and complexity of construction,operation and maintenance.

Another means of transforming the mechanical forces associated withdifferential relative movement of adjacent segments is to employ linearelectric motors such as the Lorentz type of actuator. Commonimplementations are induction motors and synchronous motors. In aLorentz type actuator, current and magnetic field strength are relatedby the equation:

({right arrow over (F)}=q{right arrow over (v)}×{right arrow over (B)})

FIG. 4 illustrates one such arrangement using magnet linear generatorssuch as that produced by ASLM, Inc. Magnets 410 and coils 420 positionedappropriately on adjacent segments transform the linear motion of thegap opening and closing into electricity. This electricity may betransmitted along connecting wires or some similar transmissionstructure 440 to propulsion mechanism 270. The electric energy may becollected and converted at converter 430. The converter may also beintegrated with a batter to store excess energy for future use.

In FIG. 4 only a single degree of freedom for the Lorentz type actuatorsis show. However, other arrangements can be configured to allow two ormore degrees of freedom using mechanism similar to a universal joint.

Lorentz style generators and actuators can be formed into circularconfigurations as illustrated in FIG. 5, and detailed view FIG. 6. Itmay be desirable to drive a small diameter rotary electric generator ora larger diameter circular motor 510. The configuration of the circularLorentz motor/generator can be configured to suit the particularengineering application. For instance, the magnet 520 and coil 530 ringsmay be oriented with their long axis perpendicular to the axis of thering as illustrated in FIG. 5 or they may be oriented coaxially. Eitherof the rings (magnet or coil) may be fixed to one of the hull segmentsand the other rotated with respect to the fixed ring. An angularvelocity amplifying gear train similar that described above drives therotating ring to improve generator efficiency. Similar to the smalldiameter rotating motor/generator, the large diameter circular Lorentztype motor generator and gear train working in conjunction with positionsensors can be used to set the angle of flexure between adjacent hullsegments. In most embodiments it will be desirable to coat the magnetsand coils in water-proof materials. In some embodiments the Lorentz typemotors take advantage of the cooling benefits of partial submersionrelieve excess heat from the generator.

The opening and closing of the gap between segments can be used togenerate power in other ways. For instance as show in FIG. 7,piezoelectric elements positioned in the gap and rigidly connected tothe end-caps of adjoining segments would experience a compression andtension as wave crests or troughs pass across the segment connectionpoint. The strain in the piezoelectric elements caused by thecompression and tension of produces an electrical current which isharvested and stored. The strain acting on piezoelectric elements can bemagnified by a mechanical device known as a flex-tensional amplifiers710. Examples of suitable flex-tensional amplifiers are those such asthat depicted in U.S. Pat. Nos. 8,154,173 and 6,629,922, both of whichare incorporated by reference into this specification. These types offlex-tensional amplifiers may be scaled up such that they are suitablefor generating the energy needs of embodiments of the present invention.

Several flex tensional amplifiers and associated piezoelectric elementscan be connected to each end cap of adjoining segments. While providingthe mechanical connection necessary between segments, the flex-tensionalamplifiers would work in concert to generate electricity as the segmentsflex due to wave induced differential movement of the segments. A radialpattern is as shown in 720 but other arrangements may be implementedthat are suitable for the geometry of the embodiment.

The electric power generated by the wave energy harvesting mechanismsemployed in embodiments of the invention is stored in a battery foronboard use. This energy is used to power the electric propulsionmechanism, but can also be used to power a navigation system, datacollection and storage, and transmission systems.

In addition to energy extracted from wave motion, additional power canbe extracted from solar energy while the vessel is operating on thesurface. Energy can be extracted from photovoltaic collectors affixed tothe upward facing surfaces of the vessel. These can be conformal withthe vessel hull (e.g. a curved surface) or mounted on hull structuresthat present a larger projected area facing upward such as a singlehorizontal surface or multiple surfaces more favorably oriented tocapture solar energy. In sea state conditions that do not support waveenergy extraction, solar energy can be collected for propulsion, storageand on-board system consumption.

At least one propulsion mechanism 270 located at end of array indirection of travel (bow). Pulling vessel through water rather thanpushing it. Multi-element, flexible line array is hard to push throughwater. A forward mounted propulsion mechanism in underwater vehicles isdescribed in U.S. Pat. Nos. 6,725,797, 6,701,862, and 6,701,862, each ofwhich is hereby incorporated by reference into this specification.

At relatively slow speeds, control surfaces for horizontal or verticalmaneuvering of a vessel have much reduced effect. As show in FIG. 8, thepropulsion mechanism may implement steerable propeller thrust 810coupled with a steerable shroud 820 that further directs the flow ofwater to impart maneuvering forces on the vessel. In one embodiment, theshroud is a circular cross section coaxially positioned in front of acylindrical vessel with a gap between the propeller shroud and thevessel. The shroud and the vessel share a similar diameter, and theshroud is open to the sea at the distal end. A propeller is positionedinside the shroud which provides a duct for improved propellerefficiency. The shroud is movably attached to the vessel in such a waythat it can be rotated about orthogonal axes; notionally a horizontaland a vertical axis.

When the orientation of the axis of the shroud is coincident with thatof the vessel, the propeller thrust is symmetrical around the perimeterof the shroud and so no asymmetric maneuvering force (e.g., up, down,left, right) is imparted on the vessel. When the shroud and duct arerotated about one or more of the axes, the direction of thrust of thepropeller is changed which imparts a maneuvering force on the vessel.When turned, the shroud opens the gap between the shroud and vessel at alocation in the direction desired to maneuver while simultaneouslyclosing the gap on the opposite side. In this way the thrust of thepropeller is preferentially emphasized.

A similar propulsion mechanism can be added at the other end of thevessel to provide increased thrust for speed, increased maneuveringforces and additional maneuvering options when running the systems inthe same or opposite direction of thrust.

The steerable cowling is capable of producing maneuvering forces to theleft, right, upward and downward when it is enabled to rotate on twoorthogonal axes. Maneuvering forces in combination with a variableballast system and autonomous control provide the means for embodimentsof the invention to change depth in a controlled fashion. There areseveral benefits of being able to operate submerged. Propulsionefficiency is increased when the cowling and propeller are fullysubmerged. A submerged vessel has the added benefit of increasedefficiency because the vessel avoids the deleterious effects of piercingthe air-water interface and potentially adverse wind conditions. Whensubmerged, a watercraft avoids exposure to potentially adverse windconditions.

Wave motion propagates downward in the water column so that a wavepowered vessel can extract energy from the environment though atdramatically reduced rates. In deep water that is deeper than thewavelength of the waves, wave induced velocity of water diminishesexponentially with depth below the surface. The relative magnitude ofthe vertical component of wave induced water velocity at a depth z (Vz)is estimated by the equation V_(z)≈e^(ω) ² ^(z/g)

Angular velocity is defined by ω=(2πg/L)̂(½) where g is the gravitationalacceleration and L is the wavelength of the wave. The chart at FIG. 9illustrates the rapid attenuation of water velocity with depth for awave with L=50 m. Displacement of the particle is proportional tovelocity so the relative attenuation of wave induced water velocity alsopredicts the relative displacement of the displacement. It is evidentthat wave “height” at a depth about 10% of the wavelength is reduced toabout 50% of the value at the surface. Because wave energy flux isproportional to the square of the wave height as previously discussed,the amount of energy that could be extracted from wave energy at a depth10% of wavelength is about 25% of that at the surface. Energy can beextracted from wave induced motion of water at depth below the surfacebut the amount of energy available drops of exponentially with depth.

With the large volume of space enclosed by the hull segments of certainembodiments of the present invention and the desire to partiallysubmerge the vehicle so that water resistance is decreased, aspreviously mentioned it would be advantageous to carry electric energystorage devices such as batteries. Batteries enable embodiments of theinvention to store energy harvested in excess of that needed forpropulsion. The stored energy is then available to provide propulsionwhen harvested wave energy was inadequate for desired transit speeds.On-board storage of energy also enables the embodiments of the inventionto maintain operation in the absence of wave energy to harvest. Thesecircumstances may occur when wave heights are very low or when thewatercraft is submerged in a water column below the effects of waveenergy on the surface. Submerging embodiments of the invention isdesirable to avoid navigation obstacles or reach particular locationsnecessary to make observations or measurement. Embodiments of theinvention may also submerge to a particular depth and even rest on thebottom to avoid heavy weather or make observations and measurements ofphenomena better observed at depth. Replenishment of stored power wouldbe accomplished returning to the surface or close to the surface wherewave energy could be harvested again.

1. An apparatus, comprising a plurality of linearly ordered segments,the plurality having two extremity segments, and wherein at least twosegments are connected; a connecting assembly joining each of the twoconnected segments of the plurality of linearly ordered segments, theconnecting assembly configured to enable the two connected segments toarticulate relative to each other in at least one degree of freedom; awave energy harvesting mechanism operatively connected to the connectingassembly that extracts mechanical energy from ambient liquid waves andconverts the mechanical energy into electrical energy; a propulsionmechanism attached to an extremity segment, operatively connected to andpowered by the wave energy harvesting mechanism.
 2. An apparatus,comprising a plurality of linearly ordered segments, the pluralityhaving two extremity segments, and wherein at least two segments areconnected; a connecting assembly joining each of the two connectedsegments of the plurality of linearly ordered segments, the connectingassembly configured to enable the two connected segments to articulaterelative to each other in at least one degree of freedom; a wave energyharvesting mechanism operatively connected to the connecting assemblythat extracts mechanical energy from ambient liquid waves and convertsthe mechanical energy into electric energy, the connecting assemblyhaving two ends, each end attached to a separate segment, and the waveenergy harvesting mechanism, further comprising a hydraulic pump havingtwo ends and containing hydraulic fluid, a first end attached to a firstend of the connecting assembly and a second end attached to a second endof the connecting assembly, and wherein each end of the hydraulic pumpcan move relative to the other such that the hydraulic fluid iscompressed or expanded; and a propulsion mechanism attached to anextremity segment, operatively connected to and powered by the waveenergy harvesting mechanism.
 3. An apparatus, comprising a plurality oflinearly ordered segments, the plurality having two extremity segments,and wherein at least two segments are connected; a connecting assemblyjoining each of the two connected segments of the plurality of linearlyordered segments, the connecting assembly configured to enable the twoconnected segments to articulate relative to each other in at least onedegree of freedom; a wave energy harvesting mechanism operativelyconnected to the connecting assembly that extracts mechanical energyfrom ambient liquid waves and converts the mechanical energy intoelectrical energy; the connecting assembly having two ends, each endattached to a separate segment, and the wave energy harvestingmechanism, further comprising a piezoelectric element operativelyconnected to a flex-tensional amplifier, the combination rigidlyconnected between each end of the connecting assembly; and a propulsionmechanism attached to an extremity segment, operatively connected to andpowered by the wave energy harvesting mechanism.